Scroll machines are becoming more and more popular for use as compressors in refrigeration systems as well as air conditioning and heat pump applications. The popularity of scroll machinery is primarily due to their capability for extremely efficient operation. Generally, these machines incorporate a pair of intermeshed spiral wraps, one of which is caused to orbit with respect to the other so as to define one or more moving chambers which progressively decrease in size as they travel from an outer suction port towards a center discharge port. An electric motor is normally provided which operates to drive the scroll members via a suitable drive shaft. During normal operation, these scroll machines are designed to have a fixed compression ratio.
Air conditioning and refrigeration systems experience a wide range of loading requirements. Using a fixed compression ratio compressor to meet this wide range of loading requirements can present various problems to the designer of the system. One method of adapting the fixed compression ratio compressors to the wide range of loading requirements is to incorporate a capacity modulation system into the compressor. Capacity modulation has proven to be a desirable feature to incorporate into the air conditioning and refrigeration compressors in order to better accommodate the wide range of loading to which the systems may be subjected. Many different approaches have been utilized for providing this capacity modulation feature. These prior art systems have ranged from control of the suction inlet to bypassing compressed discharge gas directly back into the suction area of the compressor. With scroll-type compressors, capacity modulation has often been accomplished via a delayed suction approach which comprises providing ports at various positions along the route of the compression chambers which, when opened, allow the compression chambers formed between the intermeshing scroll wraps to communicate with the suction gas supply, thus delaying the point at which compression of the suction gas begins. This delayed suction method of capacity modulation actually reduces the compression ratio of the compressor. While such systems are effective at reducing the capacity of the compressor, they are only capable of providing a predetermined or stepped amount of compressor unloading. The amount of unloading or the size of the step is dependent upon the positioning of the unloading ports along the wraps or the compression process. While it is possible to provide multiple stepped unloading by incorporating a plurality of unloading ports at different locations along the compression process, this approach becomes more and more costly as the number of ports is increased and it requires additional space to accommodate the separate controls for opening and closing each individual on each set of ports.
The present invention, however, overcomes these deficiencies by enabling an infinitely variable capacity modulation system which has the capability of modulating the capacity from 100% of full capacity down to virtually zero capacity utilizing only a single set of controls. Further, the system of the present invention enables the operating efficiency of the compressor and/or refrigeration system to be maximized for any degree of compressor unloading desired.
In the present invention, compressor unloading is accomplished by cyclically effecting axial separation of the two scroll members during the operating cycle of the compressor. More specifically, the present invention provides an arrangement wherein one scroll member is moved axially with respect to the other scroll member by a solenoid valve which operates in a pulsed width modulation mode. The pulsed width modulation operating mode for the solenoid valve provides a leakage path across the tips of the wraps from the higher compression pockets defined by the intermeshing scroll wraps to the lower compression pockets and ultimately back to suction. By controlling the pulse width modulation frequency and thus the relative time between sealing and unsealing of the scroll wrap tips, infinite degrees of compressor unloading can be achieved with a single control system. Further, by sensing various conditions within the refrigeration system, the duration of compressor loading and unloading for each cycle can be selected for a given capacity such that overall system efficiency is maximized.
The various embodiments of the present invention detailed below provide a wide variety of arrangements by which one scroll member may be axially reciprocated with respect to the other to accommodate a full range of compressor unloading. The ability to provide a full range of capacity modulation with a single control system as well as the ability to select the duration of loaded and unloaded operation cooperate to provide an extremely efficient system at a relatively low cost.
Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.